The present invention relates to a liquid crystal display device and a method of manufacturing the same.
A variety of liquid crystal display devices have been proposed as display devices for information devices. Currently, of these liquid crystal display devices, devices using nematic liquid crystals represented by the TN mode (Twisted Nematic mode) disclosed in Jpn. Pat. Appln. KOKAI Publication No. 47-11737 and the STN mode (Super Twisted Nematic mode) disclosed in Jpn. Pat. Appln. KOKAI Publication No. 60-107020 are popularly used.
In TN and STN mode liquid crystal display devices, alignment of liquid crystal molecules are twisted about 90.degree. and 260.degree. in the initial states, respectively. Light incident on a liquid crystal layer emerges with a change in polarization state due to the twisted alignment structure of the liquid crystal molecules and birefringence.
When a voltage is applied to the liquid crystal layer, the liquid crystal molecules reorient themselves in the direction of electric field, and the above-described twisted structure is lost. Consequently, birefringence is lost, and incident light emerges without any change in polarization state. More specifically, the optical characteristics of the liquid crystal layer change depending on application/nonapplication of a voltage. For this reason, when the liquid crystal device is sandwiched by two linear polarizers, a change in intensity of exit light is observed. The TN or STN mode is a display scheme of obtaining contrast in density on the basis of this operation principle.
A liquid crystal display device of such a display scheme is more advantageous than a CRT (Cathode Ray Tube) because the power consumption is much lower, and a low-profile device can be manufactured. The liquid crystal display devices are widely used in office information devices such as personal computers or wordprocessors.
However, the liquid crystal display device of the above display scheme uses polarizers and may not effectively use incident light. Many liquid crystal display devices have light sources (backlights) behind the devices to ensure exit light intensities. In a liquid crystal display device having a color filter, the light transmittance further lowers, and therefore, a more powerful light source is required.
The power for the light source almost equals the power consumption of the liquid crystal display device including a driving circuit. For this reason, the liquid crystal display device of the above display scheme is not suitable as a display for a battery-driven portable information device.
More specifically, in the liquid crystal display device of the conventional display scheme, the improvement in brightness and the reduction in power consumption are antinomic independently of color display or monochrome display.
Such a liquid crystal display device normally uses a fluorescent light as a backlight. This light is not preferable because it causes considerable eyestrain in long-time viewing on the display. A demand has arisen for development of a display scheme with high light utilization efficiency, which is applicable to a reflection liquid crystal display device requiring no backlight.
In use of a liquid crystal display device as a projection display, a compact device having a long service life and power saving of the entire device can be realized by increasing the light transmittance. Hence, for a projection liquid crystal display device as well, a demand for development of a display scheme with high light utilization efficiency has arisen.
To meet such requirements, various display schemes using no polarizers have been proposed. For example, as a display scheme using no polarizers, a scheme called NCAP (Nematic Curvilinear Aligned Phase) or PDLC (Polymer Dispersed Liquid Crystal) is known. In these display schemes, a liquid crystal layer is formed by dispersing a nematic liquid crystal material having a positive dielectric anisotropy in a polymer matrix to form droplets of the liquid crystal material having a diameter of about several micrometers. This liquid crystal material is selected such that the refractive index for ordinary rays nearly equals that of the polymer matrix, and the refractive index for extraordinary rays differs from that of the polymer matrix.
According to this display scheme, in the initial state, the liquid crystal molecules in each liquid crystal particle have a distorted alignment structure. In addition, the alignment direction changes between the liquid crystal particles. For this reason, refractive index differences are generated between most liquid crystal particles and the polymer matrix. As a result, light scattering occurs, as in frosted glass.
When a sufficient voltage is applied to this liquid crystal layer, the liquid crystal molecules reorient themselves in each liquid crystal particle, and the refractive indices of each liquid crystal particle and the polymer matrix equal with respect to light which is perpendicularly incident on the liquid crystal layer. Consequently, neither refraction nor reflection occurs on the interface between each liquid crystal particle and the polymer matrix, resulting in a transparent state. Note that the incident light need not be linearly polarized light.
In the above-described display scheme, a liquid crystal material is dispersed into a medium, unlike a display scheme using liquid crystal microcapsules (to be described later). The liquid crystal display device of this display scheme can be easily formed by encapsulating a polymer matrix dispersed with particles of a liquid crystal material in a glass cell to be used for a general liquid crystal display device or applying the polymer matrix on a substrate.
However, when the liquid crystal molecules are to be aligned by post-processing such as stretching, to increase the contrast, or a conductive polymer film is to be laminated on the liquid crystal layer, the strength of this liquid crystal layer is insufficient. In addition, the light utilization efficiency cannot be increased in color display which requires a color filter, though no problem is posed when a transparent-opaque change is produced for display or when a white-black change is produced for display by adding a black dichroic dye.
As another display scheme using no polarizers, a display scheme using a guest-host liquid crystal formed by adding a guest dychroic dye to a host liquid crystal material is known. According to this display scheme, the light transmittance is controlled by changing the alignment direction of the dychroic dye molecules. M ore specifically, a voltage is applied to the liquid crystal layer to reorient the liquid crystal molecules which aligned themselves parallel to the substrate surface in the initial state to be perpendicular to the substrate surface. By changing the direction of the dychroic dye molecules accordingly, the light transmittance is controlled.
According to this display scheme, a transparent-coloring change can be realized without using any polarizers. In addition, according to this display scheme, when cyan, magenta, and yellow liquid crystal layers are stacked via intermediate substrates, color display can be performed without using any color filter.
However, the dychroic dye has only one absorbance axis and cannot absorb a polarized light component perpendicular to the absorbance axis. Additionally, since the dychroic dye has low solubility in the host liquid crystal m at erial and low molar extinction coefficient, the above display scheme cannot realize display at high contrast.
To realize a liquid crystal display device using a guest-host liquid crystal which can display at higher contrast, various examinations have been made. For example, a White-Taylor type guest-host liquid crystal display device is disclosed in "Journal of Applied Physics (J. Appl. Phys.), Vol. 45, pp. 4718-4723 (1974)". This liquid crystal display device uses, as a liquid crystal material, a mixture of a liquid crystal compound and a dichroic dye having a chiral nematic phase. In this device, light is efficiently absorbed by the dye due to the twisted structure of the chiral nematic phase, so high display contrast can be obtained in principle without using any polarizers.
However, to achieve high contrast in this liquid crystal display device, the helical pitch in the alignment of liquid crystal molecules having the chiral nematic phase must be set on the order of the wavelength of light. If the helical pitch is decreased, many lines of disclination are formed to degrade the display quality. Simultaneously, since the hysteresis phenomenon occurs, the response speed to voltage application excessively lowers. Therefore, this liquid crystal display device is of no practical use, as compared to the above-described liquid crystal display device of TN or STN mode.
As still another display device using a guest-host liquid crystal, a liquid crystal display device using liquid crystal microcapsules is known. An examination has been made to obtain a color liquid crystal display device having a high light utilization efficiency by using the guest-host liquid crystal microcapsules. For example, a technique of preparing guest-host liquid crystal microcapsules having different absorption wavelengths and mixing them to form a liquid crystal layer (Jpn. Pat. Appln. KOKAI Publication No. 58-144885) or a technique of stacking three liquid crystal layers having different absorption wavelengths using guest-host liquid crystal microcapsules without using any intermediate substrates of glass or a plastic (Japanese Patent Application No. 7-56086) is known.
As described above, the liquid crystal display device using liquid crystal microcapsules need not use polarizers. For this reason, the light utilization efficiency increases, and high display contrast can be expected. However, the current liquid crystal display device using liquid crystal microcapsules has not achieved high display contrast yet.
This is partially because when liquid crystal microcapsules are used, all liquid crystal molecules cannot be aligned in a desired direction in the absence of applied voltage. More specifically, in the absence of applied voltage, the liquid crystal molecules in the liquid crystal microcapsules are aligned to be parallel or perpendicular to capsule walls having curved surfaces. For this reason, the liquid crystal molecules in the liquid crystal microcapsules are nonuniformly aligned. When the liquid crystal molecules are nonuniformly aligned, the light absorbance or light transmittance becomes lower than that of a structure with all liquid crystal molecules aligned in one direction. Hence, the liquid crystal display device using liquid crystal microcapsules cannot obtain high display contrast.